docs: def add_cell_markdown(self, cell_str):
          """
          Add a markdown cell
          :param cell_str: markdown text
          :return:
          """
  
          logging.debug("add_cell_markdown: {}".format(cell_str))
          # drop spaces and taps at beginning and end of all lines
          #cell = '\n'.join(map(lambda x: x.strip(), cell_str.split('\n')))
          cell = '\n'.join(cell_str.split('\n'))
          cell = nbf.v4.new_markdown_cell(cell)
  
          self.nb['cells'].append(cell)